Molecular mechanisms which regulate the intracellular bioavailability of metals such as lead and cadmium have been studied. High affinity cytosolic leadbinding proteins (PbBP) of 63,000 (63K) and 11,500 (11.5K) daltons from kidneys of rats have been purified by gel, hydrophobic interaction, and anion exchange chromatography, electrophoresis, and sucrose density gradient analysis. Cell-free nuclear translocation studies showed both time- and temperature-dependent uptake. Cd++ ions effectively blocked the nuclear uptake of 203Pb but Zn++ stimulated uptake. In vivo Pb-injection studies showed a close temporal relationship between formation and loss of Pb intranuclear inclusions in renal proximal tubule cell nuclei and marked changes in renal 2-D gel 35S-labelled protein synthesis patterns. The data indicate that these high affinity PbBP, which act as the initial cytosolic ligands for Pb in the kidney, are capable of mediating the intranuclear translocation of Pb and that the presence of Pb within renal nuclei is temporally associated with marked changes in renal gene expression. The 11.5K dalton protein, but not the 63K protein, was also found to regulate the inhibitory effects of Pb on the heme biosynthetic pathway enzyme Delta-aminolevelinic acid dehydratase (ALAD). The data indicate that the 11.5K dalton protein confers partial resistance to Pb inhibition of liver ALAD in vitro and suggests a similar role for this protein in kidney with respect to the resistance of renal ALAD to Pb inhibition. PbBP chelation of Pb and donation of Zn to the ALAD were found to be the mechanisms of this effect. Replicate studies performed with purified Zn metallothionein and purified ALAD demonstrated identical effects.